Studies with diazotized sulfanilic acid (DSA) and spirally cut strips of rabbit aorta indicated that this agent reversibly inhibited the contraction induced by norepinephrine but not that by histamine. The data suggested that inhibition was due in large part to inactivation of norepinephrine by DSA but that DSA may have in addition a selective affinity for the adrenergic receptor. Additional studies on rabbit aorta with the H1 and H2 histamine antagonists indicated that the response to histamine was mediated by both H1 and H2 receptors with H1 having the predominant effect.